Cryptocurrency marketplace NiceHash said the contents of its bitcoin wallet had been stolen in a security breach and one executive said nearly $64 million had been lost.

NiceHash head of marketing Andrej P. Škraba told Reuters on Thursday that the hack was “a highly professional attack with sophisticated social engineering” and that about 4,700 bitcoin, worth about $63.92 million at current prices, were lost, Reuters reported.

He said the company was co-operating with local authorities but declined to give more information.

Earlier, Slovenia-based NiceHash said in a statement posted on its website that it had halted operations for 24 hours as it investigated the breach.

NiceHash is a digital currency marketplace that matches people looking to sell processing time on their computers in exchange for the digital currency bitcoin.

NiceHash advised users to change online passwords, saying it was still trying to understand what had happened during the incident.

“We understand that you will have a lot of questions, and we ask for patience and understanding while we investigate the causes and find the appropriate solutions for the future of the service,” the company said in its statement.